花灑d水既問題

It is simply that the surface of the shower head where the water is ejected is not perfectly flat. As such, water coming out from the tiny holes is inclined in a small angle.

When the shower head is held in a downward position, water ejecting out would be attracted by gravity to fall vertically once leaving the holes, despite the samll inclination angle during ejection.

But when the shower head is held upward, the up-and-down motion of the water stream would manifest the ejction angle considerably, and hence the water stream appears as parabolic.